1. Field of the Invention
The present invention generally relates to thin film optical coating technologies. In particular, the present invention relates to electrically tunable thin film optical devices where the light propagates in a direction perpendicular to the plane of the thin film and fabrication methods therefor.
2. Description of the Related Art
Thin film optical coating technology is used in a wide variety of optical applications such as antireflection coatings, mirrors and filters. These coatings are static in that the thickness and index of refraction are determined at the time of manufacture. Therefore, the optical properties are fixed at the time of manufacture.
The electro-optic effect of the modulation of the index of refraction of a material upon application of an electric field is well known. Another contribution to the optical path associated with application of an electric field to a thin film is the piezoelectric, or electro-striction, effect in which the length of the material changes upon application of an electric field. These wave propagation in electro-optic and piezoelectric material effects are summarized in equations (1)–(5) shown below:{right arrow over (E)}={right arrow over (E)}0e−i(ωt−{right arrow over (k)}·{right arrow over (z)})  (1)                                                      k                                ->                =                                                            2                ⁢                                                                  ⁢                π                            ⁢                                                                                  λ              0                                ⁢          n                                    (        2        )                                                      Δ            ⁢                                                  ⁢            ∅                    π                =                              2                          λ              0                                ⁡                      [                                          n                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                z                            +                              z                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                n                                      ]                                              (        3        )            Δz/z=deffEappl(4)                              Δ          ⁢                                          ⁢                      n            /            n                          =                                            n              2                        2                    ⁢                      r            eff                    ⁢                      E            appl                                              (        5        )            where Equation (4) is for the piezoelectric effect and Equation (5) is for the electro-optic effect.
Upon application of an electric field, Eappl, the total optical phase change, Δθ is dependent on the electro-optic effect and the piezoelectric effect. The equations shown above determine the phase change as a function of the applied electric field.
FIG. 1 shows a structure 100 in which an electro-optic (and/or piezoelectric) material can be made to modulate light. The structure of FIG. 1 is transverse and in the plane of the thin film. The structure is transverse in that the electric field is applied transverse to the direction of propagation, which is in the plane of the sample as indicated by the optical wave vector, k, as is commonly used in electro-optic applications today.
Because the light propagates in the plane of the thin film, light must be coupled into and out of this plane, which can be inconvenient. Also, the number of devices which can be made, per wafer of thin film material, is small. Finally, since the light is propagating in the plane of the thin film, it is difficult to add more thin film optical elements, such as mirrors and other electro-optic layers to manufacture more complex integrated optical devices.